The standard NTSC color television broadcast system adopted in the United States by the Federal Communications Commission provides a six megahertz bandwidth for each television broadcast channel. Within this six megahertz bandwidth, broadcasters must provide video, color and sound information together with several synchronizing signals. The latter are required to enable the television receiver to properly display the picture information as originally modulated at the transmitter.
The use of conventional information processing by television broadcast and receiving systems involves the modulation of analog information signals upon one or more carrier signals for broadcast. This analog signal processing together with the limited bandwidth available for each television broadcast channel limits the maximum frequency response of picture information which may be transmitted and received. This limited frequency response results in a limitation of the resolution o picture definition of the displayed television picture. Within these constraints, manufacturers of television receivers have adopted a variety of picture information processing and display techniques to enhance the perceived definition or resolution of the displayed television picture.
While the effort by television manufacturers to provide such enhanced resolution has provided considerable improvement, there exists a continuing desire to further improve television picture quality. This continuing desire has motivated practitioners in the television arts to undertake the development of a variety of high definition television systems. One of the fundamentals applied to such high definition television system development is the need to process higher frequency picture information. One possible solution involves expanding the available bandwidth of the television broadcast channels. While such a system is technologically direct, its implementation is considered impractical given the proliferation of standard NTSC signal stations and television receivers. Another possible approach involves compressing a wideband video signal while maintaining the present six megahertz broadcast channel bandwidth. In this regard, transmission of the compressed signal in digital form is generally preferred since improved noise performance is provided relative to conventional analog transmission. That is, any noise picked up in the digital transmission of the encoded signals will not be reproduced if the discrete levels of the digital transmission can be accurately resolved in the receiver. However, in digital transmission, excessive noise or interference can result in total loss of picture if the digital levels cannot be resolved, in contrast to analog where the picture is gradually degraded.
While the structures of high definition television systems may vary, a typical wideband system uses a high frequency analog video source having a bandwidth in excess of thirty megahertz for picture information. The broad bandwidth video information is converted from analog to digital information by conventional analog to digital conversion. Thereafter, digital signal processing techniques are utilized to perform one or more data compression operations to provide a signal capable of being digitally transmitted within the available six megahertz bandwidth. This digitally encoded signal is used to modulate an assigned television broadcast carrier.
At the receiver, a generally conventional tuner, intermediate frequency amplifier, and picture detector recover the digitally encoded signal in much the same manner as a conventional NTSC receiver. Thereafter, decompression systems produce a representation of the original compressed signal after which a digital to analog converter converts the digitally encoded signal to a wideband analog picture information signal. This analog signal is then processed for display upon a high resolution television display such as a cathode ray tube.
Such high definition television systems will, of course, be incompatible with standard NTSC systems. Because of the great number of NTSC television receivers presently installed and in use, it is likely that the implementation of such high definition television systems will be carried forward in a manner which avoids disturbing FCC allocation of existing NTSC terrestrial services. One of the most likely approaches to making terrestrial high definition television available involves assigning presently unused television broadcast channels to high definition television broadcasting. Through the years, the Federal Communications Commission has endeavored to avoid assigning the same television broadcast channels to broadcasters operating in potentially overlapping television service areas. As a result, most areas in the United States have a number of unused television channels available. However, the assignment of such channels to high definition television broadcasters will increase the likelihood of closely spaced or overlapping broadcast areas. In such areas, television receivers may be subjected to two different television broadcast signals on the same broadcast channel. The result of simultaneous reception of two different signals within the same broadcast channel produces an interference problem within the receiver known as co-channel interference. High definition television receivers receiving digitally transmitted signals may be subject to stronger interference signals than receivers receiving NTSC signals and as a result such co-channel interference signals are likely to severely degrade or even disrupt the operation of the high definition television receivers.
Another form of co-channel interference occurs in cable television systems due to a phenomenon known as composite triple beat. A typical cable television distribution system includes a plurality of amplifiers spaced at regular intervals between a cable head-end and remote subscriber locations. The distribution system may accommodate eighty or more 6 MHz television channels in a band between about 50-550 MHz. In processing television signals transmitted over these channels, the distribution amplifiers develop numerous beat components comprising the combination of two or more of the transmitted signals. Of particular concern are the so-called composite triple beats which result from the combination of signals transmitted over three separate channels since these beat components tend to fall within the bandwidth of the transmitted cable spectrum. Moreover, due to the numerous possible combinations, any given cable channel may be exposed to composite triple beats from a large number of sources along the length of the cable plant, the accumulation of which provides an interfering co-channel signal whose average power is largely concentrated at the carrier frequencies of the given cable channel. Cable television channels located near the center of the cable spectrum may, for example, experience co-channel interference resulting from the accumulation of up to about 1,000 different composite triple beats while channels near the opposite ends of the spectrum experience co-channel interference resulting from the accumulation of up to about 500 different composite triple beats.
Composite triple beat induced co-channel interference establishes a major design limitation in current cable systems which may impact the introduction of recently proposed compressed NTSC transmissions. In particular, in order to increase the channel capacity of existing cable television systems it has been proposed to transmit up to four or more compressed NTSC television signals over a single 6 MHz cable television channel. While the composite triple beat induced co-channel interference caused by the additional channels will be somewhat ameliorated by the use of low-level digital transmission technology much the same as has been proposed for high definition television, some adverse co-channel interference effects are nevertheless anticipated.
Since the channel selection tuning mechanisms used in television receivers provide channel selection based upon signal frequency, even the best tuner may not exclude an undesired co-channel signal. There arises, therefore, a need in the art for an effective means for reducing co-channel interference, both in digital high definition television receivers and in cable television signal receivers, especially those designed for receiving digitally encoded compressed NTSC signals.
Accordingly, it is a general object of the present invention to provide an improved television signal receiver. It is a more particular object of the present invention to provide an improved television signal receiver capable of reducing the effects of NTSC co-channel interference without significantly degrading the performance of the receiver.